Many electrical circuits and systems require the use of a power resistor to perform various functions such as, for example, establishing desired voltage and current levels for associated circuitry and/or to divert electrical power from another electrical device. One example of the latter use is in known automotive air conditioning systems which typically utilize a power resistor to control the speed of an air conditioning blower motor. In certain operational modes, the power resistor may be used to divert a considerable amount of power from the blower motor into the incoming air stream. Due to such high power dissipation, the power resistor typically operates at temperatures of between approximately 80-150 degrees C.
In many of the foregoing electrical circuits and systems, potential failure modes exist wherein the power resistor may become excessively hot due to high current flow therethrough. Such excessive heat may cause thermal damage to surrounding circuitry and structures, and possibly result in a fire. To circumvent the possibility of hazardous thermal conditions, such power resistors are typically equipped with a thermally activated fuse designed to open circuit the resistor when the operating temperature thereof rises to some predefined temperature range.
Designers of electrical circuits and systems have heretofore devised a variety of approaches in providing thermally fused electrical components, particularly with respect to film-type electrical components formed on a substrate. One such approach involves the use of a spring loaded metal cantilever connected, typically via solder, between the electrical component and a terminal thereof. When the temperature of the electrical component increases to within a predefined temperature range, the solder attachment between the component and cantilever melts and the spring loaded cantilever pulls away from the component to create an open circuit condition. An example of this approach is shown in U.S. Pat. No. 3,638,083 to Dornfeld, et al.
Although the foregoing approach has been successfully demonstrated, it is inherently unreliable. For example, over time, temperature cycling of the component due to normal operation causes the solder connections to weaken until the spring loaded cantilever pulls away from the component, thereby resulting in an open circuit condition.
Another common approach for providing a thermally activated fuse, particularly for use with a film-type electrical component, is shown in FIG. 1. Referring to FIG. 1, a pair of conductive circuit paths are formed on one side of a substrate 14, and a so-called thick film electrical component 16, which may be a resistor, is formed therebetween in accordance with known techniques. A first component terminal 18 may be electrically connected to circuit path 10 and a second component terminal 20 may be electrically connected to a third conductive circuit path 22 formed adjacent to circuit path 12. A thermally activated fuse element 24 is then electrically connected between circuit paths 12 and 22.
As an alternative to the arrangement of FIG. 1, yet another common approach for providing a thermally activated fuse, particularly suited for use with a film-type electrical component, is shown in FIG. 2. Referring to FIG. 2, a pair of conductive circuit paths 30 and 32 are formed on one side of a substrate 34 with a thick-film electrical component 36 formed therebetween. On the opposite side of substrate 34, a pair of conductive circuit paths 38 and 40 are formed in alignment with circuit paths 30 and 32 respectively. A first component terminal 42 may be electrically connected to circuit paths 30 and 38, and a second component terminal 44 may be electrically connected to circuit paths 32 and 40. A thermally activated fuse element 46 is then electrically connected between circuit paths 38 and 40 opposite electrical component 36.
In the thermally activated fuse approaches shown in FIGS. 1 and 2, fuse elements 24 (FIG. 1) and 46 (FIG. 2) may typically be meltable wires, attachable conductive links designed to fall off, or solder paste designed to reflow, when the operating temperature of the electrical component increases to a predefined temperature range. With each of these known fuse structures, however, several problems are known to exist. For example, in the case of a meltable wire, the wire may melt but may not pull away from circuit paths 38 and 40 sufficiently to break the electrical connection. In the case of attachable conductive links, which are typically attached to circuit paths 38 and 40 via solder, the solder may melt but the conductive link may not come away from the circuit to open circuit the component 36. This problem is compounded if the component 36 is not properly oriented. Finally, in the case of solder paste, such paste tends to lose its liquid component over time, and further due to temperature cycling, so that it may not properly melt and pull away from circuit paths 38 and 40 and open circuit the electrical component as desired. Examples of some of the various thermal fuse arrangements shown and described with respect to FIGS. 1 and 2 are shown in U.S. Pat. No. 4,494,104 to Holmes, U.S. Pat. No. 4,533,896 to Belopolsky and U.S. Pat. No. 5,084,691 to Lester, et al.
Another problem associated with each of the foregoing known thermal fuse arrangements is an inherent inaccuracy in opening the fuse element, and correspondingly open circuiting the electrical component, when the operating temperature of the electrical component reaches an excessive temperature range. As shown in FIGS. 1 and 2, the fuse elements 24 and 46 are positioned remotely from the heat generating component. For example, as shown in FIG. 1 fuse element 24 is positioned adjacent electrical component 16, and as shown in FIG. 2, fuse element 46 and electrical component 36 are positioned on opposite sides of the substrate 34. In each case, regardless of the type of fuse structure used, the electrical component must heat the entire substrate to an excessive temperature range before the fuse opens. In order to do so, the operating temperature of the electrical component, typically a resistor, will therefore rise above the temperature at which the fuse opens. This phenomenon is shown in FIG. 3 which shows a plot of resistor temperature 47 and fuse temperature 48 versus time. As illustrated in FIG. 3, if the fuse element is not in intimate contact with the resistor surface, the maximum temperature of the resistor, T.sub.R,MAX, increases to a temperature level above the fuse opening temperature, T.sub.F, by an amount .DELTA.T before fuse opening occurs.
The foregoing problem with known thermally fused electrical components which is illustrated in FIG. 3 may have several undesirable effects. For example, the additional resistor temperature increase .DELTA.T may be sufficient to cause combustion of nearby structures. Further, excessive heating of the entire substrate may cause damage to unrelated circuitry and/or other structure in close proximity thereto.
What is therefore needed is a thermally fused resistor arrangement that reliably open circuits the heat generating resistor when the operating temperature thereof reaches an excessive level. Such a thermal fuse should ideally be placed in intimate thermal contact with the resistor so that it opens as soon as the operating temperature of the resistor reaches a predefined temperature range. An optimum placement of such a thermal fuse should, in fact, correspond to the so-called hot spot of the resistor which, as the term is used herein, is defined as the region of the resistor generating maximum heat.